The cause of their deaths was smoke inhalation, and the manner of their deaths was ruled undetermined, according to the Clark County coroner's office.

If convicted of the charges Hatzenpiller and Gouailhardou could face 10-year to life prison terms on each murder count and 2- to 20-year prison terms on the child abuse and neglect counts.

The children were sleeping in their home when the fire started about 10:20 a.m., fire officials said.

Hatzenpiller, the mother of Skylar and Aryana, and Gouailhardou, David's father, escaped the fire.

Neighbors tried to rescue the children but could not. Neighbors at the time questioned how the parents could leave their children behind.

By the time firefighters had the blaze under control, the children were dead, and two mobile homes were destroyed.

Prosecutor David Stanton said agents with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were instrumental in identifying the potential cause of the fire. Clark County fire investigators and Las Vegas police also investigated the case.

The federal agents think the fire was caused by a smoldering cigarette but acknowledge there is a small probability it might have been caused by one or more of the children playing with a lighter.

Transcripts of the grand jury investigation will be made public in the coming weeks. The indictment lists Special Agent Daniel Heenan as a witness who testified to the grand jury.

Heenan is considered a national expert in fire investigations in law enforcement circles and has trained police agencies throughout the country and around the world, including Scotland Yard.

The indictment shows that two employees of the Clark County Department of Family Services were called to testify to the grand jury, including Assistant Director Paula Hammack and a caseworker.

Hatzenpiller had a lengthy history of contact with the agency before the fire, records show.

In April, child welfare workers investigated an abuse allegation involving Hatzenpiller's children. They ruled the case unsubstantiated because the alleged perpetrator did not care for or have custody of the children, according to the report. The case was closed May 7.

Hatzenpiller faced four neglect allegations in 2005 and 2006 involving another child. Three were investigated and deemed unsubstantiated by child welfare workers. The fourth allegation came from out of state, and county investigators closed the case because they could not find the family.

In June 2011, child protective services received an abuse allegation involving David Gouailhardou. But the information in the allegation was not enough to start an investigation, and the case was closed as "information only," according to the report.

Shane Gouailhardou had contact with the agency in 2006 as a 16-year-old victim of neglect. The allegation was substantiated, and he was made a ward of the county until his family completed its case plan in 2009.

Gouailhardou, who has a history of minor drug arrests, was charged in August with possession of methamphetamine and has a case pending in North Las Vegas Justice Court, records show.

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